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We should gene-sequence cave paintings to find out more about who made them

Standard archeological techniques can’t determine whether the prehistoric artists were hunter-gatherers or farmers. Analyzing the paintings’ surfaces with techniques from biology offers much more information about how they worked and when they lived.

Some of the great wonders of the artistic world are the cave paintings in southern Europe, particularly in eastern Spain. This rock art is thought to have been created between 5,000 and 8,000 years ago, when human societies were making the transition from hunter-gatherer to farming communities.

Despite much study, the origin of these artworks is shrouded in mystery. Nobody is quite sure what the artists used for paint or binder, how the pigmentation has been preserved for so long, and—most controversial of all—exactly when the images were made. In particular, archaeologists would dearly love to know whether the images date from the Neolithic period, before the transition to farming, or the Mesolithic period, when the transition had already begun.

Today we get a unique insight into this question thanks to the work of Clodoaldo Roldán at the University of Valencia in Spain and colleagues, who study prehistoric Spanish Levantine rock art. This team has carried out the first genomic analysis of the bacterial communities that flourish on the rock art and of the pigment and binders that make up the images. And their work offers vital clues to to the way the work must have been created and preserved.

Eastern Spain has over 700 sites of prehistoric rock art, collectively known as Levantine art. The images are thought to be the most advanced from this period and generally depict small human figures and animals.

One way to date ancient artifacts is with carbon dating. But this works only with pigments that have a biological origin, and with the exception of black, most of them do not. That’s one reason there is widespread disagreement over dates.

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Roldán and co take an entirely different approach. They used a sterile scalpel to take tiny scrapings from the surface of the art. These samples include some pigment, its binding material, and any bacteria on the surface. They also took scrapings from blank rock that had been recently exposed because of rock fall.

The samples were tiny: each scraping consisted of less than 20 milligrams. This made the analysis challenging. Nevertheless, the researchers successfully used high-throughput sequencing techniques to reveal a huge diversity of bacteria on the rock art.

Some of these bacteria are thought to have a protective effect. For example, organisms from the bacillus genus produce oxalic acid, which produces a thin film of calcium oxalate on the rock, protecting any pigment beneath. Roldán and co say these bacteria were common in the samples.

The sequencing techniques also revealed a wide range of proteins in the pigment, including bovine albumin and casein.

That’s an important result. One theory for the way these rock paintings were made is that ancient painters mixed the pigment into cow butter and then smeared it over the rock walls. The discovery of bovine albumin and casein is entirely consistent with this idea.

That has other implications. If the images were created with cow butter, that could only have been possible in communities that had domesticated cows. In other words, the paintings must have been produced in Mesolithic communities that had begun to farm, and not in Neolithic communities based on hunting and gathering.

Of course, it is conceivable that the art was contaminated with cow butter over the years—there is no way to rule out this possibility using sequencing techniques.

However, if the art contains biological materials, radiocarbon dating should be possible. So the new work opens the possibility that this technique should work, provided a big enough sample can be obtained.

That’s interesting research that shows how modern sequencing techniques are beginning to influence archaeology. Expect to hear more about them.